Iminium Salts Research Articles

Synthesis of novel β-aryl-β-(methylthio)acroleins via Vilsmeier–Haack protocol as potential 1,3-dielectrophilic three-carbon building blocks

A new general route for the synthesis of novel β-aryl-β-(methylthio)acroleins, a class of stable potential 1,3-dielectrophilic synthons, has been reported. The overall protocol involves treatment of either β-chloroacroleins or their precursor iminium salts (generated in situ from the corresponding active methylene ketones under Vilsmeier–Haack reaction conditions) with S,S-dimethyldithiocarbonates (DDC)/aqueous KOH in either a one-pot or two-step process. The dimethyldithiocarbonate (DDC)/30% aqueous KOH has been shown to be an excellent source of methylthiolate anion.

The Reaction of Cyanamidium Salts with Ylidenecyanamide Derivatives

Cyanamidium salts 1undergo ene reactions with ylidenecyanamide derivatives 5to afford conjugated iminium salts 12. The N,N,Nʹ-trialkylcyanamidium salts 1react as the ene, and the ylidenecyanamide derivatives 5react as the enophile components to form the 2-azoniaallene salts 11 followed by the formation of conjugated iminium salts 12 as cationic polynitrogen compounds with guanidine and amidine subgroups. The constitution of the new conjugated iminium salts 12 was secured by elemental analyses and spectroscopic data (IR and NMR).

Preparation and Structures of 2‐Substituted 5‐Benzyl‐3‐methylimidazolidin‐4‐one‐Derived Iminium Salts, Reactive Intermediates in Organocatalytic Transformations Involving α,β‐Unsaturated Aldehydes

AbstractPreparations of the title compounds, 5–7 (Scheme 1 and Table 1), of their ammonium salts, 9–11 (Scheme 2 and Table 2), and of the corresponding cinnamaldehyde‐derived iminium salts 12–14 (Scheme 3 and Table 3) are reported. The X‐ray crystal structures of 15 cinnamyliminium PF6 salts have been determined (Table 4). Selected 1H‐NMR data (Table 5) of the ammonium and iminium salts are discussed, and structures in solution are compared with those in the solid state.

Homogenous Pd-catalyzed asymmetric hydrogenation of unprotected indoles: scope and mechanistic studies.

An efficient palladium-catalyzed asymmetric hydrogenation of a variety of unprotected indoles has been developed that gives up to 98% ee using a strong Brønsted acid as the activator. This methodology was applied in the facile synthesis of biologically active products containing a chiral indoline skeleton. The mechanism of Pd-catalyzed asymmetric hydrogenation was investigated as well. Isotope-labeling reactions and ESI-HRMS proved that an iminium salt formed by protonation of the C═C bond of indoles was the significant intermediate in this reaction. The important proposed active catalytic Pd-H species was observed with (1)H NMR spectroscopy. It was found that proton exchange between the Pd-H active species and solvent trifluoroethanol (TFE) did not occur, although this proton exchange had been previously observed between metal hydrides and alcoholic solvents. Density functional theory calculations were also carried out to give further insight into the mechanism of Pd-catalyzed asymmetric hydrogenation of indoles. This combination of experimental and theoretical studies suggests that Pd-catalyzed hydrogenation goes through a stepwise outer-sphere and ionic hydrogenation mechanism. The activation of hydrogen gas is a heterolytic process assisted by trifluoroacetate of Pd complex via a six-membered-ring transition state. The reaction proceeds well in polar solvent TFE owing to its ability to stabilize the ionic intermediates in the Pd-H generation step. The strong Brønsted acid activator can remarkably decrease the energy barrier for both Pd-H generation and hydrogenation. The high enantioselectivity arises from a hydrogen-bonding interaction between N-H of the iminium salt and oxygen of the coordinated trifluoroacetate in the eight-membered-ring transition state for hydride transfer, while the active chiral Pd complex is a typical bifunctional catalyst, effecting both the hydrogenation and hydrogen-bonding interaction between the iminium salt and the coordinated trifluoroacetate of Pd complex. Notably, the Pd-catalyzed asymmetric hydrogenation is relatively tolerant to oxygen, acid, and water.

ChemInform Abstract: Unconventional Stereoselective One‐Pot Synthesis of Knoevenagel‐Type Indoles via in situ Condensation of Iminium Salts with Active Methylene Reagents.

AbstractEt3N‐mediated condensation of preformed or in situ generated indole iminium salts with active methylene reagents provides stereoselectively Knoevenagel‐type indoles in moderate to good yields.

Quinap and congeners: atropos PN ligands for asymmetric catalysis.

Among the range of P,N-chelating ligands that have been employed in asymmetric catalysis, those relying on atropisomerism for the stability of individual enantiomers form a definable class. These APN (atropos P,N) ligands require a specific type of biaryl, with one component carrying a pendant phosphine unit, most commonly diaryl substituted, and the other bearing an sp(2)-nitrogen adjacent to the biaryl link. When substituents in the biaryl inhibit rotation about the linking bond, stable nonracemizing six-membered ring chelates can be formed. This Perspective relates the background to the initial synthesis in 1993 of Quinap, the original member of the series, and initial observations on its effectiveness in asymmetric catalysis. The current state of play in development of syntheses of this and other members of the APN ligand family is assessed, and their applications in asymmetric catalysis are presented. These include hydroboration and diboration of alkenes, 1,3-dipolar cycloadditions, alkynylation of iminium salts in a three-component (A(3)) condensation, and conjugate additions of Cu acetylides.

Propyne Iminium Salts and Isoquinoline – 1 : 1 and 2 : 1 Adducts

The reaction of equimolar amounts of propyne iminium trifluoromethanesulfonates 1a, b and isoquinoline yielded, after hydrolytic work-up, the N-(3-oxoprop-1-en-1-yl)isoquinolinium salts 4a, b in modest yields. Monitoring of the reaction by 1H NMR spectroscopy indicated the formation of salts 4, 3-isoquinolinio-substituted propene iminium salts 3, and N,N,N',N'-tetramethylvinamidinium salts 5 as the major components. The expected aminoallenes (2-(3-(dimethylamino)allen-1- yl)isoquinolinium triflates) 2 could not be detected in the reaction solutions. It is possible, however, to trap the aminoallene intermediates in a polar [4+2] cycloaddition reaction, as shown by the isolation of 2 : 1 adducts 7c, d in good yield from cyclopropyl-substituted propyne iminium triflates 1c, d and isoquinoline. Hydride abstraction from 7c, d yielded the 2,4-dicyclopropyl-1,3- bis((dimethyliminio)(aryl)methyl)pyrido[2,1-a]isoquinolinium tris(triflates) 8c, d.

A Simple Metal-free Synthesis of 2,4,5-Trisubstituted Pyridines and Pyridine N-Oxides by [2+2] Cycloaddition of Enaminones to Propyne Iminium Salts

Herein a simple one-pot metal-free synthesis of 2,4,5-trisubstituted pyridines and pyridine Noxides by [2+2] cycloaddition of enaminones, which are prepared in situ from alkyl, aryl and heteroaryl methyl ketones using N,N-dimethylformamide dimethyl acetal (DMFDMA), and propyne iminium salts as electron-poor acetylenes, is described.

Thiaminpyrophosphat – ein natürlich vorkommendes Iminium-Salz. Seine Bedeutung für die mikrobiellen Prozesse bei der Weinbereitung und die Aromabildung im Wein / Thiamine Diphosphate – A Natural Iminium Salt. Its Role in Microbiological Processes During Wine Preparation and on the Aroma of Wine

Iminium salt structures are widespread in nature and play an important role in biochemical processes. Among these thiamine diphosphate (ThPP, Vitamine B1) is one of the best known examples. It serves as a cofactor in a variety of different enzymes that are found in all forms of life. During fermentation for the production of alcoholic beverages, ThPP is of great importance not only for the key step (decarboxylation of pyruvate), but also for the formation of secondary metabolites which highly influence the aroma of wine. Thus, enzymatic degradation of amino acids in yeasts via the Ehrlich pathway delivers higher alcohols, fatty acids and esters. In malolactic fermentation, ThPPdependant enzymes (ligases) form new C-C bonds to synthezise the highly aroma-active substance diacetyl. This article summarizes the influence of ThPP on the sensorically important substances during wine making processes.

Tribenzylamine C–H Activation and Intermolecular Hydrogen Transfer Promoted by WCl6

The 1:1 molar reaction of WCl6 with tribenzylamine (tba), in dichloromethane, selectively afforded the iminium salt [(PhCH2)2N═CHPh][WCl6], 1, and the ammonium one [tbaH][WCl6], 2, in equimolar amounts. The products were fully characterized by means of spectroscopic methods, analytical methods, and X-ray diffractometry. Density functional theory calculations were carried out with the aim of comprehending the mechanistic aspects.

Synthesis and chemosensitivity of a new iminium salt toward a cyanide anion

A short, high-yielding route to pyranylidene Iminium (Imi) salts using a new pyrylium salt reaction between N,N-Dimethylformamide (DMF) and acetic anhydride is reported. The Imi salt-sensing behavior toward various anions has been investigated using UV–Visible spectroscopy. The Imi salt demonstrates high selectively for CN− when various other anions, such as CN−, Cl−, Br−, I−, SCN−, ClO4-, NO3-, HSO4-, PF6- and N3-, are present because it is highly reactive towards nucleophiles. The selective detection of CN− with the Imi unit gave rise to a significant hypochromic shift in the CH3CN solution at λmax=444nm and 423nm and creation of new peak at 252nm. These studies indicated that CN− had high affinity toward Imi, forming a 1:1 complex; this observation agrees with the current understanding of these materials.

Palladium-Catalyzed Stille-Type Coupling of N-Acyl Iminium Ions with Distannanes: A Multicomponent Synthesis of α-Amidostannanes

The palladium-catalyzed three-component coupling of imines, acid chlorides, and hexabutyldistannane is described. This results in the synthesis of various α-amidostannanes in good yields, without the use of strong nucleophilic organometallic reagents. The reaction is believed to proceed via a Stille-type cross coupling of an in situ-generated N-acyl iminium salt with Bu3SnSnBu3 and reaches completion within 1 h at ambient temperature using simple, unligated Pd2dba3 as a catalyst. Combining the formation of α-amidostannanes with lithiation and carboxylation allows the overall synthesis of amino acid derivatives in two steps from imines, acid chlorides, and CO2.

ChemInform Abstract: A Novel Domino Condensation—Intramolecular Nucleophilic Cyclization Approach Towards Annulated Thiochromenes.

AbstractA domino reaction of isoquinoline‐derived iminium salts and α‐mercapto benzaldehydes provides access to previously unknown annulated thiochromenes.

Chemistry of Iminium Salts, Imines, and Related Compounds

Chemistry of Iminium Salts, Imines, and Related Compounds

The frustrated Lewis pair pathway to methylene phosphonium systems

Reaction of HB(C6F5)2 with (tBu3C6H2)P(vinyl)2 does not yield the expected ethylene-bridged frustrated Lewis pair (FLP) but its cyclic methylene phosphonium isomer which is formed in a unique way by subsequent internal 1,2-P/B addition to the vinylphosphane unit. The product is a rare example of a stable phosphorus analogue of the ubiquitous iminium salts. It features a short PC bond (1.637(3) Å). Subsequent reactions with pyridine and with dihydrogen probably proceed by equilibration with the “invisible” ethylene-bridged FLP isomer. The reaction of (tBu3C6H2)P(vinyl)2 with B(C6F5)3 gave the zwitterionic acyclic methylene phosphonium product that was crystallized at low temperature and isolated. The X-ray crystal structure analysis also revealed a short PC bond (1.635(5) Å).

ChemInform Abstract: Synthesis of New Mono‐N‐tosylated Diamine Ligands Based on (R)‐(+)‐Limonene and Their Application in Asymmetric Transfer Hydrogenation of Ketones and Imines.

AbstractRuthenium catalysts (I) with chiral diamine ligands are successfully applied to the asymmetric transfer hydrogenation of ketones, imines, and iminium salts.

Unconventional stereoselective one-pot synthesis of Knoevenagel-type indoles via in situ condensation of iminium salts with active methylene reagents

A simple one-pot procedure for the stereoselective synthesis of Knoevenagel-type indoles is described. The method is based on the in situ reaction of indole iminium salts (four of them are fully characterized) with acyclic symmetrical and unsymmetrical active methylene reagents in the presence of triethylamine. In general, the overall yields are moderate to good. Some of relevant reaction parameters and steric effects affecting stereoselectivity are discussed.

ChemInform Abstract: Integrated Reactions Using Addition to Conjugated Imines and Iminium Salts

AbstractReview: 18 refs.

Orthoamides and Iminium Salts, LXXXVI [1]. Synthesis and Characterization of <I>N</I>-<I>ω</I>-Aminoalkyl-Substituted Guanidines and Their use for CO<SUB>2</SUB> Capturing

Orthoamides and Iminium Salts, LXXXVI [1]. Synthesis and Characterization of <I>N</I>-<I>ω</I>-Aminoalkyl-Substituted Guanidines and Their use for CO<SUB>2</SUB> Capturing

Kinetic resolution in asymmetric epoxidation using iminium salt catalysis.

The first reported examples of kinetic resolution in epoxidation reactions using iminium salt catalysis are described, providing up to 99% ee in the epoxidation of racemic cis-chromenes.